Disclosed herein is a downhole seal element, and related apparatuses.
In the art of downhole tools, i.e. devices intended to be conveyed into water, oil and gas wells and similar elongate boreholes extending into subterranean formations, various types of seal element are known. They are of particular utility when used to convey tools into wells that are normally filled with a pumped, circulating fluid the chemical composition of which will vary from one well to the next.
Such a seal element typically is made from a flexible, and commonly resiliently deformable, material and includes a collar that normally lies at an in-use downhole (leading) end of the seal. The collar is sealingly secured on the exterior of some part of a downhole toolstring, typically a mandrel, that almost invariably is circular. The seal element includes a skirt that extends in use uphole (rearwardly) from the collar. By reason of the flexible nature of the material of the element the skirt is capable of moving from a collapsed position lying close to the mandrel to an expanded position flared outwardly therefrom.
The mandrel is of a smaller diameter than e.g. drillpipe temporarily defining the inner wall of the well in which it is to be conveyed. The skirt when collapsed while being of greater diameter than the mandrel nonetheless also is of a lesser diameter than the inner wall defined by the drillpipe.
The skirt may be caused to move from its collapsed to its extended position by the application of fluid pressure inside the drillpipe, with the pressure gradient acting in the downhole direction. As a result it is possible to employ one or more seals to cause movement of a downhole tool along a length of drillpipe in a fluid-filled well, as long as (a) the seal element is mounted the correct way round on the tool and (b) the circulation of fluid is such as to apply fluid pressure to the skirt in a desired direction causing the skirt to expand so that its outer periphery seals against the drillpipe. Since at this time both the innermost part of the seal element represented by the collar and the outer periphery of the skirt define seals the pumping of fluid in the well causes the tool supporting the seal element to be conveyed in a desired (normally downhole) direction.
In some cases such conveying of a tool is adequate for the purpose of deploying it from a surface location to a subterranean location. Following the completion of the intended action of the tool it may be recovered to the surface location for example by paying out a cable that may attach to the uphole end of the deployed tool using a per se known fishing neck arrangement. The cable may then be wound in to the surface location in order to recover the tool.
Such an approach is often acceptable when the tool in use is essentially autonomous. In many situations however the use of an autonomous tool is not possible.
One example of a non-autonomous tool is a wireline logging tool.
A logging tool is an elongate, cylindrical device that is conveyed to a downhole (operational) location for the purpose of logging (i.e. recording, processing and/or analyzing) data about the subterranean formation.
Wireline is a form of armored cable that is capable of transmitting electrical and electronic signals from the logging tool to a surface location. Many designs of logging tool are conveyed to their downhole locations trailing a length of wireline behind them so that log data may be telemetered immediately to an uphole location and analyzed.
Wireline offers numerous advantages in many logging situations but it is characterized by having a comparatively high mass per unit length. In some situations wireline must be paid out over a length of several thousand or even tens of thousands of feet in order to let a tool reach the total depth of a well. This means that many hundreds of kilograms of wireline may lie in the well while logging takes place.
If the well extends vertically or steeply downwardly the mass of the wireline is not seen as a particular disadvantage because gravity tends to avoid the need to apply additional energy in order to deploy it. In other words in such wells the mass of the wireline tends to be no hindrance to tool deployment.
Many wells however are not of this character, and extend horizontally (for example sideways into a hillside) or at least include sections that are not vertical or steeply descending. In such situations a need arises to pump the logging tool along the well in the manner outlined above using seals as aforesaid. When pumping under these circumstances is required the mass of the wireline becomes a significant problem because much energy is then needed to move the logging tool (which itself may weigh more than a hundred kilograms) and the wireline. This additional energy normally takes the form of an increase in the pumping pressure of the fluid circulating in the well. The pumping pressure is controlled by a logging engineer stationed at the surface location.
Furthermore the wireline and/or the tool may become snagged or impeded in some way, and at such times high pumping pressures again are employed in order to try and move the logging tool.
These factors create limits to the extent to which wireline logging tools can be pumped in wells. The limits arise either because the pumps used to circulate fluid in the wells are not capable of creating sufficiently high pressures or (more commonly) because existing seal elements when subjected to high pressures tend to fail by turning “inside out” with the result that their skirts cease to seal against the inner wall of drillpipe or casing in the well. When this happens the seal becomes useless for its intended purpose of pumping the tool; and indeed the seal may become torn or broken up such that it merely is debris inside the drillpipe.
In view of the foregoing there is a need for an improved design of seal arrangement that in particular is suitable for use when a heavy mass of wireline must be pumped along a well together with the logging tool.